1. Field of the Invention.
The present invention relates to a bipolar integrated circuit amplifier. In particular, the present invention is an improved amplifier circuit having a cascode amplifier stage.
2. Description of the Prior Art.
The use of a cascode stage to broad band an amplifier stage is commonly used and practiced in integrated circuit design. This configuration, however, is prone to instability and becomes more susceptable to oscillation with increased current levels which are desirable in low noise applications.
One typical amplifier having a cascode amplifier stage is a differential amplifier having a pair of transistors with their emitters connected together and their bases connected to receive the two input signals. The collector of the two differential amplifier transistors are connected to the emitters of the cascode stage transistors. The bases of the cascode stage transistors are connected to a bias voltage. The collectors of the cascode stage transistors are connected through resistors to a supply line.
In the prior art, attempts have been made to stabilize the cascode amplifier stage of the differential amplifier circuit by including resistors in the emitters of the cascode transistors or by using compensation capacitors between the supply line and the collectors of the cascode transistors. Both of these prior art methods decrease the frequency response of the amplifier circuit. In addition, emitter resistors increase the differential input capacitance of the differential amplifier transistors. Low input capacitance is extremely important in applications where the driving source of the amplifier is an inductive load, such as a read/write head of a magnetic storage device.